The present invention relates to a cannulated screw for orthopedic bone surgery and particularly relates to a self-drilling, self-tapping cannulated screw designed to minimize or eliminate the danger of thermal necrosis, afford efficient chip removal both during insertion and removal, and generally facilitate insertion of the screw into the bone and its removal.
Many and various types of supposed self-drilling screws have been designed and used in the past. Many of these self-drilling screws require pre-drilling before insertion of the screw or the screw must be run in and out multiple times at progressively increasing depths before the screw can be finally seated. The latter is a result of chip loading on the screw during insertion causing the self-tapping screws to load up with chips and preventing further insertion of the screw into the bone. Moreover, upon attempted further insertion, oftentimes the threads will strip. Resorting to multiple in-and-out movements of the screw relative to the bone also results in wearing the threads, which increases the tendency for inaccurately placed bore holes and the danger of heat build-up during use.
Many such screws are designed with problems of screw insertion in mind but neglect the problems of removing the screw from the healed bone. It will be appreciated that the bone, upon healing, grows about the screw and thus it is necessary to withdraw the screw similarly as when inserting the screw by cutting the bone.
In accordance with the present invention, there is provided a cannulated, self-tapping, self-drilling screw, preferably for orthopedic bone surgery, which includes a shaft having tip, head and self-tapping threaded and shank portions. The screw is cannulated, i.e., it has an axial bore through the entire length of the screw opening through the head and tip portions. The screw includes a pair of cutting points (more than two cutting points may be provided) which are designed to self-drill to provide a proper tap hole ahead of the advancing self-tapping thread design of the threaded portion. The points feature a free cutting action reducing heat build-up at the cutting site and afford efficient chip rejection and removal, eliminating chip loading problems which can interfere with the ease and timeliness of the pin insertion. A pair of side flutes are formed in the screw threaded portion, the ends of the flutes terminating in the cutting points. The outer surfaces of the screw between each of the points and the first of the self-tapping screw threads extend circumferentially about the screw to terminate at a circumferentially adjacent flute, such outer surfaces forming an angle of between about 5.degree.-7.degree. with the axis of the screw. It has been found that this angle is critical and preferably is about 6.degree.. A larger angle than, for example, about 7.degree., slows the cutting action and creates greater heat. A lesser angle, for example, of less than 5.degree., fails to clear the chips, necessitating multiple runs in and out. Hence, this slightly conical outer surface, which is interrupted by the flutes, has been found to be most effective in reducing heat and carrying chips away from the cutting site. The distal end of the tip also has surfaces which face in the axial direction of the screw. These surfaces extend from the cutting point, preferably at an angle of about 17.degree., circumferentially about the screw to terminate at the circumferentially adjacent cutting edge at a location axially inwardly of the point of the adjacent cutting edge. This facilitates the cutting action with minimal heat build-up.
The threaded portion of the screw also carries at least a pair of reverse cutting flutes at circumferentially spaced positions thereabout and spaced axially back from the flutes adjacent the tip portion. The cutting edges of the reverse cutting flutes are arranged to cut through the bone upon unthreading the screw from the bone. The reverse cutting edges start at an axial location adjoining the smooth shank of the screw whereby the reverse cutting edges tap the bore hole in which the screw resides during unthreading of the screw. The curved surfaces on the cutting edges of the flutes assist in removing the chips and funneling the chips into the threads for efficient removal and prevention of lock-up due to chip compaction. The head of the screw is provided with external flats, i.e., preferably in a hexagonal configuration, such that a tool may be applied externally about the head for both threading and unthreading rotations.
In a preferred embodiment according to the present invention, there is provided a cannulated self-tapping, self-drilling screw for orthopedic bone surgery comprising an elongated shaft having an elongated axis and tip, head, self-tapping threaded, and shank portions, the screw-threaded portion being disposed adjacent the tip portion and the shank portion disposed between the head and the screw-threaded portions. The shaft defines a cannula extending coaxially through the shaft and opening through the tip and the head portions. A plurality of flutes are formed in the screw-threaded portion at circumferentially spaced positions thereabout and interrupt the threads thereof to define self-tapping cutting edges along the interrupted threads for threading the screw into the bone, the tip portion including a pair of circumferentially spaced cutting points at the distal end of the tip portion, the distal end of the tip portion having end surfaces facing generally axially of the screw with each end surface extending from a cutting point at a predetermined angle circumferentially about the screw to terminate at a circumferentially adjacent cutting edge at a location axially inwardly of the point of the adjacent cutting edge. The outer surface of the screw between each point and the first of the self-tapping screw threads of the tip portion extends circumferentially about the screw, terminating at a circumferentially adjacent flute and forming an angle of between about 5-7 degrees with the axis of the screw.
In a further preferred embodiment according to the present invention, there is provided a cannulated self-tapping, self-drilling screw for orthopedic bone surgery comprising an elongated shaft having an elongated axis and tip, head, self-tapping threaded, and shank portions, the screw-threaded portion being disposed adjacent the tip portion and the shank portion disposed between the head and the screw-threaded portions. The shaft defines a cannula extending coaxially through the shaft and opening through the tip and the head portions. A plurality of flutes are formed in the screw-threaded portion at circumferentially spaced positions thereabout and interrupt the threads thereof to define self-tapping cutting edges along the interrupted threads for threading the screw into the bone. The tip portion includes a pair of circumferentially spaced cutting points at the distal end thereof, the outer surface of the screw between each point and the first of the self-tapping screw threads of the tip portion extending circumferentially about the screw and forming an angle of between about 5-7 degrees with the axis of the screw. At least a pair of reverse cutting flutes are disposed in the threaded portion of the screw, the reverse cutting flutes being circumferentially spaced one from the other, spaced axially from the plurality of flutes and interrupting the threaded portion to define reverse cutting edges for cutting bone upon unthreading the screw from the bone.
Accordingly, it is a primary object of the present invention to provide a novel and improved self-tapping, self-drilling cannulated screw for orthopedic bone surgery which minimizes or eliminates the danger of heat build-up at the cutting site, affords efficient chip removal, eliminates chip loading problems which can interfere with the ease and timeliness of screw insertion and in general facilitates a quick one-step screw insertion and removal capability.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.